Mammal ancestry expanded after dinosaurs died off

Feb. 8, 2013
|

An artist's rendering of the hypothetical placental ancestor, a small, insect-eating animal. The research team reconstructed the anatomy of the animal by mapping traits onto the evolutionary tree most strongly supported by the combined phenomic and genomic data and comparing the features in placental mammals with those seen in their closest relatives. / Carl Buell

by Dan Vergano, @dvergano, USA TODAY

by Dan Vergano, @dvergano, USA TODAY

A half-pound, bug-eating critter looks like the common ancestor of most modern mammals, biologists suggest.

The end of the Age of Dinosaurs 65.5 million years ago appears to have led to the plethora of mammal types springing from this distant ancestor, the team led by Maureen A. O'Leary of Stony Brook (N.Y) University reports. Scholars have debated for the last two decades whether the explosion of mammal types in the distant past, leading to everything from people to elephants, began before or after the dinosaurs disappeared.

Mammals are defined as the warm-blooded, furred animals distinct from reptiles, birds and bugs. The mammals in the study do not include marsupials, mammals that carry their young in a pouch such as kangaroos, which spring from a more ancient split in the mammal family.

"Species like rodents and primates did not share the Earth with non-avian dinosaurs but arose from a common ancestor - a small, insect-eating, scampering animal-shortly after the dinosaurs' demise," O'Leary says, in a statement.

Comparing physical characteristics across 86 mammal species, living and extinct, and cross-checking with genetic data to create age estimates, the Science journal study concludes that the original shrew-like ancestor of mammals with a placenta sprang up from earlier mammals within 400,000 years of the end of the Age of Dinosaurs. And the species likely first sprang from older marsupial-like mammal types in South America and North America, rather than in Africa, given the geologic timing of events.

Genetic evidence alone had pointed to an older origin for most non-marsupial mammals, while fossils pointed to an earlier one. The new analysis adds to evidence favoring the more recent origin for the plethora of mammal varieties now seen on Earth.